How to Divide Perennials

April 11th, 2018

Dividing plants is an easy way to expand your garden flock at no charge.

Dividing perennials not only contains the spread of growing perennials, but it gives you free plants.

It works best for perennial flowers — the ones that come back year after year (that is, when deer, rabbits and groundhogs don’t eat them).

Basically, you:  a.) Dig up the whole clump, b.) Separate the clump into two or more fist-sized pieces, and c.) Replant the divisions as you would a new plant.

Ta-daaa! You’ve now got free new plants that are exact copies of what you started with. Trade with neighbors to diversify both of your yards.

See George’s video on how to divide perennials

Doesn’t this risk killing the plants?

It sounds ruthless, but so long as you separate big-enough pieces (i.e. fist-sized or more) and keep the planted pieces watered afterward, the survival odds are very high.

Division actually reinvigorates many older perennials and reduces disease. Younger sections replanted from the spreading perimeter usually bloom nicely the next season.

When’s the time to divide?

Early spring and early fall are two excellent times for most species. That’ll give the roots several weeks to establish at a lower-stress time than in the heat of summer.

In general, divide perennials when they’re not blooming. Fall bloomers such as mum, aster, sedum, goldenrod and Japanese anemone are best divided in spring. Spring bloomers such as creeping phlox, foamflowers, salvia, sweet woodruff and dianthus are best divided in early fall.

Most summer bloomers can go either way.

How do I get the pieces apart?

Some plants simply pull apart. You’ll be able to grab a section of stems and pull them — roots and all — apart from the mother.

Examples are blackeyed susans, coneflowers, hardy geraniums, lamium, Shasta daisies, salvia, foamflowers and, most of the time, daylilies, liriope and asters.

Other plants have roots that are tightly interwoven, so you’ll have to use a shovel, knife or even an ax to split these clumps.

Examples: ornamental grasses, iris, astilbe, hosta, sedum, yarrow, goats beard, mums, coreopsis, dianthus, ferns and catmint.

Any I shouldn’t try to divide?

Baptisia is an example of a deep, tap-rooted perennial that doesn’t divide very well.

Perennials with deep, single tap roots and ones that have habits similar to woody shrubs don’t divide. These include baptisia, Russian sage, butterfly weed, baby’s breath, artemisia, lavender, columbine and euphorbia.

Anything else I need to do?

Work compost into the soil before replanting. That’ll improve drainage and add fresh organic matter and nutrition.

Replant divisions at the same depth as the mother plant.

Scratch a little balanced, granular fertilizer into the soil at planting, and water twice a week until the ground freezes whenever rain doesn’t do the deed for you.

Two good books on dividing and caring for perennial flowers:

“The Well-Tended Perennial Garden” by Tracy DiSabato-Aust (Timber Press, 2006, $34.95 hardcover)

“The Perennial Care Manual” by Nancy J. Ondra (Storey Publishing, 2009, $24.95 paperback)